The present invention relates to a toroidal continuously variable transmission, and more particularly to a ratio control system for a toroidal continuously variable transmission.
Recently, the use of toroidal continuously variable transmissions as motor vehicle transmissions has been highlighted again owing to development of traction oil with a high traction factor.
The toroidal continuously variable transmissions include coaxial input and output discs having facing toric surfaces, and a plurality of friction rollers, which may be called as power rollers, disposed between and in driving contact with the toric surfaces for transmitting a rotary motion between the input and output discs. For a ratio change, the friction rollers incline from an initial position to a new desired position. A known toroidal continuously variable transmission of the above kind is disclosed in JP-A 63-225754.
According to this known transmission, a plurality of friction rollers are rotatably supported by roller carriers, respectively, by eccentric shafts, respectively. Each of the roller carriers is supported on a housing by swivel joints for rotation about a carrier axis thereof and for limited movement along the carrier axis. Shifting the roller carriers along their carrier axes causes roller inclination of the associated friction rollers.
The axial displacement of each of the roller carriers is brought about by a hydraulic actuator supplied with hydraulic fluid from a ratio control valve. The roller inclination is determined by the amount of and direction of axial displacement of the associated roller carrier which in turn is controlled by the hydraulic fluid pressure via the associated hydraulic actuator.
The ratio control valve for the hydraulic actuators includes a sleeve moveable along a valve axis, and a spool slidably received by the sleeve for relative movement to the sleeve. A stepper motor is drivingly connected to the sleeve to change the sleeve from an initial position to a new desired position. A precess cam is provided on one of the roller carriers for converting a rotary motion of the roller carrier to a linear transational motion of the spool of the ratio control valve. The roller inclination is transmitted to the ratio control valve in this manner. The ratio control valve regulates supply of hydraulic fluid to and discharge thereof from the hydraulic actuators in response to movement of the sleeve relative to the spool.
From the preceding description, it will now be understood that the translation of the spool follows the translation of the sleeve, producing a desired roller inclination and thus a desired ratio position corresponding to a command supplied to the stepper motor by a control unit.
The above-mentioned known transmission suffers from a problem that an error in its feedback system is difficult to correct.
An object of the present invention is to improve a toroidal continuously variable transmission such that a deviation in its feedback system owing to manufacturing and/or assembling tolerances is easy to correct.